Author Archives: Michael Bloom

Buffalo Bayou and Tributaries Study (Again)

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I posted about this study back in June of 2019, when the United States Army Corps of Engineers (USACE) was asking for up front public input on the study before they got too far along. I helped the Houston Branch of the American Society of Civil Engineers (ASCE) provide some input.

Back then, many regional stakeholders chimed in, most supportive of additional federal investment to update the original federally authorized project that built the Addicks and Barker Dams. Many stakeholders encouraged USACE to be creative and to evaluate nature-based systems to reduce flood risks.

At the urging of the Harris County Flood Control District (the most likely local sponsor of any federally funded project to update the Buffalo Bayou federal project) last month the USACE released an interim version of the report for public input. An executive summary of the report is available here. For more detail, the whole report is here.



The main stakeholder reaction? Disappointment.

There are two main reasons for the public’s disappointment.

First, the report includes very traditional detention and conveyance alternatives along with buy-out options. Nothing in the report could be described as creative. There is not much in the way of nature-based solutions. The use of tunnel conveyance facilities were ruled out due to cost.

Second, most of the presented alternatives have very low benefit cost ratios (BCRs). This is very important to note because in order to attract federal support (congressional authorization) and funding (congressional appropriation), projects must have BCRs of much more than 1.0. This is embedded in laws and policies governing the White House Office of Management and Budget (OMB). Limited federal dollars most compete with scores of other projects across the country, each with calculated BCRs. Only the projects with the highest BCRs have a chance at federal funding.

So why did USACE publish a report with these two problems?

Here is my theory.

After Hurricane / Tropical Storm Harvey dropped its unprecedented rain amounts on the Buffalo Bayou system, many upstream and downstream homes and businesses were flooded. This triggered a public outcry and litigation over the federal government’s use of private property to store and convey water. It also triggered support for the resilience study and created expectations that the study would find a “silver-bullet” that would “protect” everyone from a Harvey type event in the future.

Well, the situation is not favorable to identifying projects with high benefit cost ratios (BCRs). The vast majority of homes and businesses in the Buffalo Bayou system have a very, very low likelihood of flooding during any particular time period, even with Atlas 14 rainfall used to map floodplains. This low existing risk is a direct result of the large federal investment to build the existing Addicks and Barker system.

Yes, there are homes in the inundation pools of Addicks and Barker. Yes, there are homes in the floodplain of Buffalo Bayou downstream. Yes, the Cypress Creek overflow does make drainage and land use challenging upstream. But even counting them in, it is still very challenging to identify any infrastructure investment in the Buffalo Bayou watershed that would generate a high enough monetary value of avoided damage — the benefit part (the numerator) in the BCR — to justify the required costs.

To explain this further, let’s dive into how the monetary value of avoided damage is estimated. The monetary value of avoided damage is determined by the difference between the value of avoided damages from a particular flood after a new project is built, call it post-project conditions (ADPost) minus the value of avoided damages from a particular flood before a new project is built, call it pre project conditions (ADPre) with the result multiplied by the likelihood of that particular extreme flood occurring during the study time period. If the avoided damage arising from a particular flood with a new project is not much different from the “no new project” alternative, the difference won’t be very large. This is the numerator in the BCR. If the project costs are high (Cost), the BCR denominator will be high, thus reducing the BCR. Shown as an equation, it looks like this:

Where: BCR is the benefit cost ratio; ADPost is the monetary value of avoided damages after any proposed new project; ADPre is the monetary value of avoided damages today, without any new investment in the existing Addicks and Barker system; P is the probability of the modeled extreme storm occurring during the study time period; and Cost is the cost to design, build, operate, and maintain the project during the study time period.

So let’s pretend we can devise a project that generates $8 billion in avoided damages (ADPost) from a particular extreme, but rare, storm event. Because of the prior investment in the Addicks and Barker system, the ADPre is also a pretty high number, easily $6 billion. This means the difference is about $2 billion in this hypothetical.

To account for the probability of the rare extreme event occurring during the study time period, we need to multiply the damage estimate by the likelihood of that extreme storm actually occurring during the study time period. Let’s assume that over a 100 year period the rare and extreme event has a 10% chance of occurring. That means we would have to multiply the difference in damage estimates, $2 billion, by 0.10 to compute the benefit portion of the BCR fraction. Ten percent of $2 billion is $200 million.

So to obtain a BCR of 1 or more, if the benefit portion of the fraction (the numerator) is $200 million, we need the total cost to design, permit, build, and operate the project for 100 years (the denominator) to be $200 million or less. If you’ve read the report’s executive summary, or know anything about how much infrastructure costs, you know that is pretty much impossible. (The dam safety projects – spillway fixes – should obviously be done as soon as possible. Those smaller projects cost less and yield high benefits.)

Folks who flooded and who live in the Buffalo Bayou watershed, who really want an additional federal project investment to further reduce flood risks in their area may be upset with this post. But they should know that the 1940 “Definite Plan” was a very large federal investment that reduced their flood risks when it was built. The prior federal risk reduction investment makes it harder to justify additional risk reduction investments.

Unrelated to the watershed-specific study discussed in this post, additional local, state, and federal investments should go to areas of Harris County with many more homes, structures, and businesses exposed to higher inundation risks than those in the Buffalo Bayou watershed. Think Greens Bayou, Halls Bayou, and others.

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Harris County Community Flood Resilience Task Force

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1972 VERSION

The original Harris County Flood Control Task Force was created in 1972. Members of the original task force included representatives of environmental conservation groups, development and real estate interests, engineering organizations, and economic development organizations. Task force membership did not include regular citizens, flood victims, housing advocacy groups, or representatives of low income or minority groups.

NEW VERSION

Back in July of 2020 Harris County Commissioners Court, at the urging of the county’s chief elected official – Harris County Judge Lina Hidalgo, initiated a process to reboot the old task force. The new version of the task force, which will be called the “Harris County Community Flood Resilience Task Force” (emphasis added), will include regular citizens, flood victims, housing advocacy groups, and representatives of low income and minority groups. The word community has been inserted into the title to signal that this group won’t consist of only experts or industry groups.

The new Task Force By-Laws outline membership qualifications. The by-laws require that each member of the Commissioners Court appoint one person. Those five appointed members must then select and vote upon the remaining 12 members by the end of 2020. The 12 members must meet certain qualification requirements. The final by-laws define slightly different membership provisions than those defined in the July proposal. I prepared a table that illustrates the similarities and differences in the composition of the task force members. As of this writing, the first five appointed members are:

The Task Force by-laws indicate the following task force objectives (paraphrasing a bit here for simplicity):

  • Evaluate overall approach to strengthening flood resilience;
  • Evaluate projects;
  • Evaluate schedules and compliance with the equity based prioritization framework;
  • Help identify funding;
  • Provide oversight to resilience planning; and
  • Assist with community engagement.

INFRASTRUCTURE RESILIENCE TEAM

On September 15, 2020, in a 3 to 2 vote, the Commissioners Court established another entity called the Infrastructure Resilience Team (IRT). The IRT includes managers and practitioners from the following Harris County Departments and entities:

The activities of the IRT will be coordinated by Dr. Paula Lorente, formerly with Texas A&M University, who was hired by the Office of the County Engineer. The September 8, 2020 letter to Commissioners Court requesting the creation of the IRT mentions the development of a 2050 Flood Resilience Plan for the county. It also outlines the budgetary needs and staffing needs for the IRT. I encourage folks interested in additional details about the IRT to read the court letter.

In general the IRT and the Task Force will work together to implement the the Harris Thrives Resolution adopted by Commissioners Court with a 3 to 2 vote on August 27, 2019.

EXPECTATIONS

I anticipate that the IRT and the Task Force will be able to effectively work together to advance flood resilience in our community, but it will take time. Flood risk reduction and resilience enhancement work is challenging, expensive, and is needed across the entire 1,777 square miles of our county. Some estimate that a total investment of $60 billion would be needed to reduce the annual risk of inundation for all structures in the county to 1% or less.

The IRT has the expertise to plan and design projects, estimate costs, and build them, however, elected officials and their agents (like the Task Force) must reach consensus on which projects to build, where to build them, when to build them, and, more broadly, how to invest limited public funds so we can reduce flood risks and realize other community benefits at the same time. Consensus building takes time, so I anticipate that the work of the IRT and Task Force will take longer than we think it will. I urge all participants and observers to listen to each other and to be patient with each other as our community does this important work.

Review of Harvest Green

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I recently took a drive through Harvest Green, a master planned community located in Richmond, Texas. Currently being developed by the Johnson Development Corporation it features agriculture and gardening as a central theme. Amenities include a working farm; optional backyard raised bed gardens; a trail system; herb gardens in some common areas; lakes; and a fitness center with pool, splash pad, and playground. I toured the community to review its drainage system, and amenities, and to see how the two were integrated.

The master planned community will eventually occupy 1,300 acres; however, only the first few sections have been completed thus far. This is an illustration of the complete master plan.

Masterplan obtained from online development literature. https://www.harvestgreentexas.com/masterplan

In some locations the four lane main roads drain to depressed areas planted with native grasses and shrubs that provide some detention and retention of stormwater (see below). I was unable to tell if these areas subsequently drain directly to underground storm sewers and then to a detention basin or if the drainage flows through a bioretention system first. I was also unable to tell if including these areas allowed the development to proceed with a lower volume of centralized detention. [If anyone involved in the design of this project knows, please leave a comment.]

Photograph by M. Bloom.

Each of these low areas has educational signage identifying it as a “Native Meadow” (see below). This helps residents and visitors, who may be used to a more formal and manicured type of landscaping, know that the tall prairie grass and wildflowers are intentional and are part of the “natural” and “wild” Harvest Green experience and brand.

Photograph by M. Bloom.

Many of the homes have backyards adjacent to trail or creek corridors. The photograph below shows a trail along Oyster Creek. Homes along this corridor have an open fence design along back lot lines. I did not notice if gates were provided for easy home owner access to the trail system. Landscaping includes a mix of both mowed turf grass and unmowed “wild” areas with native grasses and flowers.

Photograph by M. Bloom.

Some creek corridors don’t currently include trails and are not served by backyard gates. The photograph below shows an example of this. This is a bit of a missed opportunity to integrate both natural amenities and natural drainage into these corridors.

Photograph by M. Bloom.

While some of the main roads appear to drain to natural systems, local residential streets do not. All of the local streets appear to be served by traditional curb and gutter drainage systems with underground pipes flowing to centralized detention basins. The basins were built deeper than required for detention purposes and were clay lined so that they hold some permanent water and serve as amenity lakes.  The photograph below shows the traditional raised curb and a pair of side-by-side curb inlets. As noted previously, the more extensive use of natural drainage systems can reduce the need for end-of-pipe detention systems. I was not able to tell if Harvest Green was able to take advantage of that benefit.

Photograph by M. Bloom.

A recycled water system has been added to the community’s wastewater treatment system and recycled water lines have been installed throughout the development to be used for landscape and farm irrigation. State rules require the recycled water piping system be purple and signage be installed to reduce the risk of people drinking the recycled water. The quality of the water is perfectly safe for watering food crops, but it is not intended for direct human consumption. See photograph below.

Photograph by M. Bloom.

Like most suburban communities in the region, the entry points are marked with large, illuminated, entry monuments and signage.  The photograph below shows one of the entry monuments to Harvest Green.

Photograph by M. Bloom.

But this monument is a bit unusual. It is solar powered! The photograph below shows the photovoltaic cells and what I assume is a battery system of some kind.

Photograph by M. Bloom.

The Johnson Development Corporation and their design team used natural drainage approaches to manage the runoff from some areas of the master planned community. They integrated amenities and drainage system in some areas. They deployed non-potable water reuse systems and solar systems to improve the sustainability of the development.

Their marketing of the community has focused on the natural amenities, agricultural features, and gardening elements to differentiate their offering to home buyers, some of whom are looking for a more active and engaged lifestyle and to live in a more sustainable and environmental friendly community.

The Johnson Development Corporation (and their planning and design professionals) should be commended for what they have accomplished in Harvest Green.

Buffalo Bayou and Tributaries Resiliency Study

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In the aftermath of Hurricane Harvey, the federal government appropriated $6 million and authorized the U. S. Army Corps of Engineers (USACE) to conduct the Buffalo Bayou and Tributaries Resiliency Study.

According to the USACE, the study will: Identify and evaluate the feasibility of reducing flood risks on the Buffalo Bayou, both upstream and downstream of Addicks and Barker Reservoirs in Harris County, Texas, while simultaneously completing a Dam Safety Modification Evaluation (DSME) on the two dams. Three primary problems will be addressed: (1) Flooding downstream of the reservoirs on Buffalo Bayou; (2) Performance and risk issues related to flow around and over the uncontrolled spillways; and (3) Flooding upstream of the reservoirs.

Map of the study area. The Cypress Creek watershed is included only to evaluate the overflow from that watershed into Addicks. Brays Bayou will not be considered during the development of risk reduction options but it will be considered when determining potential adverse impacts.

The Corps requested public input on the scope of the study and comments were due on May 31, 2019.

I helped coordinate the development of comments on behalf of the Houston Chapter of the Environment & Water Resources Institute of the American Society of Civil Engineers. The text of the submitted comments is provided below:

The Houston Branch of the Texas Section of the American Society of Civil Engineers appreciates the opportunity to comment on the above referenced resiliency study.  Our comments are provided below.

  1. Sustainable Infrastructure: Alternatives should be evaluated using the Institute for Sustainable Infrastructure’s ENVISION rating system.  Alternatives with the highest score in the rating system should be considered further for implementation.  See sustainableinfrastructure.org for additional information about the rating system.
  2. Non-Stationary Climate: Alternatives should be developed to handle rainfall amounts that have a 1% annual chance (or greater) occurring in the year 2100.  Rainfall depths appear to be trending upwards and the 1% annual chance event will likely be larger at that time.
  3. Nature-Based Alternatives: Alternatives should be developed and evaluated that include nature-based approaches, such as land acquisition and preservation, wetland creation, natural stable channel design approaches, and similar concepts.
  4. Two-Dimensional Modeling of Non-Riverine Areas: Alternatives should be evaluated using 2-D modeling approaches, especially in areas not adjacent or near bayous or channels.
  5. Triple-Bottom-Line Net Cost/Benefit Estimations:  Alternatives should be evaluated using a more comprehensive assessment of net benefits and costs. Net costs should be estimated for traditional engineering economics inputs, such as construction costs, operations costs, maintenance costs, land acquisition costs, and labor cost.  But environmental costs should be estimated as well. These should include the value of any diminished ecosystem services, lost habitat, lost carbon sequestration, lost oxygen production, lost heat island mitigation, lost recreational opportunities, and similar well studied metrics.  Social costs should also be estimated for each alternative. These should include displaced cultural or historical features, lost recreational opportunities, lost or diminished employment opportunities, diminished views and character, light pollution impacts, diminished social equity, and similar aspects. Net economic, social, and environmental benefits should also be estimated for each alternative.  These would include the value of avoided property damage (times the likelihood of loss), the number of people benefiting from a reduced risk of inundation, the value of any increase in social values or benefits (recreation, views, safety, equity), the value of any increase in environmental values or benefits (habitat, ecosystem services, etc.).  The net present value of all economic, social, and environmental BENEFITS minus the net present value of all economic, social, and environmental COSTS should be calculated for all alternatives and the alternative with the highest net present value of total triple bottom line NET BENEFITS should be recommended for implementation.

Again, we appreciate the opportunity to comment on the scope of the study.  If there are any questions about our comments, please don’t hesitate to contact us.

Very truly yours,

AMERICAN SOCIETY OF CIVIL ENGINEERS – HOUSTON BRANCH

Leave a comment about what you or your organization thought the study should consider.

Still Here

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Someone recently mentioned that they had not seen a riparianhouston.com post in a long time. They asked if I was still blogging or if they had somehow fallen off the subscription list.  I have not posted in a long time.

Well, I am still here and I do plan to write more posts.

Some possible future topics include:

  1. Thoughts about the current Buffalo Bayou and Tributaries Resiliency Study being conducted by the U.S. Army Corps of Engineers.
  2. My upcoming presentation at the annual meeting of the Green Infrastructure Leadership Exchange.
  3. Highlights from the final reports from the Greater Houston Flood Mitigation Consortium.
  4. How Harris County and Harris County Flood Control District plan to incorporate consideration of Atlas 14 into their development rules.
  5. Flood bond equity considerations.
  6. An update on how Harris County Engineering Department is moving forward with the use of the Envision sustainable infrastructure rating system on projects.

Leave a comment if you have other ideas.

Texas Senate Interim Report on Hurricane Harvey Response

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The Texas Senate Committee on Agriculture, Water & Rural Affairs recently released their Interim Report to the 86th Legislature in response to Lieutenant Governor Dan Patrick’s 2017 Hurricane Harvey Response Interim Charges, which were:

  • Study and make recommendations on how to move forward with water infrastructure projects in the State Water Plan that will help mitigate floods through flood control, diversion, and storage projects. Evaluate plans for a possible third reservoir in addition to Addicks and Barker to control and alleviate additional flooding in the region. Additionally, review the current status of reservoir projects in Texas. Examine opportunities for coordination between federal and state agencies to develop flood mitigation infrastructure, and the ongoing maintenance and restoration of critical dam infrastructure.
  • Study and identify ways to improve the capacity and maintain the structure of the Addicks and Barker Reservoirs. Report on mechanisms that would ensure the public has access to timely and transparent release figures from reservoirs across the state.
  • Evaluate current state data-sharing standards for rainfall and stream gauges and whether regional flood management projects and flood warnings should be hosted in a centralized location, such as a state agency web page. Determine whether a statewide real-time flood warning system could be developed and coordinated through mobile devices, TxDOT electronic signage, communication devices and whether existing local and regional forecasting infrastructure could be integrated into a centralized inclement weather forecasting system.

Chairman Perry states in his cover letter that he “will introduce legislation detailing the creation of a State Flood Plan and the funding component.”

Key selected recommendations contained in the report include the following (italics are direct quotes):

  • Update flood models;
  • Expand the role of an existing agency or create a new oversight process for the Texas Flood Plan by centralizing all local flood prevention plans. This would be like the State Water Plan;
  • Help communities achieve improved Community Rating System (CRS) scores;
  • Study deepening Addicks and Barker Reservoirs, diversion channels, and bayous;
  • Task the Texas Water Development Board to track and report all available funding (from all sources) for flood mitigation strategies in a publicly accessible manner;
  • Create a State Infrastructure Fund with an investment from General Revenue and/or the Economic Stabilization Fund to support flood mitigation strategies detailed in the State Flood Plan;
  • Review reservoir operation manuals to reflect land development, debris levels, and the science of how accumulations and stream flow will affect reservoir and floodgate capacity;
  • Create more reservoir capacity where possible by heightening walls and removing debris by dredging;
  • Support local government partnering with the federal government to increase capacity and update reservoir operations; and,
  • Create a system like the AMBER Alert program, or other means … for local officials to adequately communicate a flood evacuation with residents.

Highlights from WRDA 2018

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On October 23, 2018, the President signed Senate Bill No. 3021, America’s Water Infrastructure Act of 2018, into law (Public Law 115-270).  The law, also known as the Water Resources Development Act (WRDA) of 2018, includes some interesting provisions that I will highlight in this post.

The Sense of Congress

The first provision reaffirms that Congress would like to pass a water resources development bill each time they meet. The first WRDA was passed in 1974 and in the 1980’s and 1990’s new legislation was passed almost every two years.  Between 2000 and 2014 the pace of reauthorizations slowed. The passage of the 2018 reauthorization sets Congress back on the every-two-years pace, with bills passed in 2014, 2016, and 2018.

Move Civil Works to Another Agency?

The law requires the U.S. Army Corps of Engineers (USACE) to hire the National Academy of Sciences to conduct a two-year study on:

The ability of the Corps of Engineers to carry out its statutory missions and responsibilities, and the potential effects of transferring the functions (including regulatory obligations), personnel, assets, and civilian staff responsibilities of the Secretary relating to civil works from the Department of Defense to a new or existing agency or subagency of the Federal Government, including how such a transfer might affect the Federal Government’s ability to meet the current statutory missions and responsibilities of the Corps of Engineers; and,

Improving the Corps of Engineers’ project delivery processes, including recommendations for such improvements, taking into account factors including: the effect of the annual appropriations process on the ability of the Corps of Engineers to efficiently secure and carry out contracts for water resources development projects and perform regulatory obligations; the effect that the current Corps of Engineers leadership and geographic structure at the division and district levels has on its ability to carry out its missions in a cost-effective manner; and the effect of the frequency of rotations of senior leaders of the Corps of Engineers and how such frequency affects the function of the district.

Economic Analysis Methods

The statute also requires the USACE to hire the National Academy of Sciences also to:

carry out a study on the economic principles and analytical methodologies currently used by or applied to the Corps of Engineers to formulate, evaluate, and budget for water resources development projects; and, make recommendations to Congress on potential changes to such principles and methodologies to improve transparency, return on Federal investment, cost savings, and prioritization, in the formulation, evaluation, and budgeting of such projects.

Transparency and Stakeholder Engagement

The law requires USACE to enhance transparency and information exchange. The USACE must issue project proposal guidance for non-Federal project sponsors, assist non-Federal interests with researching and identifying USACE project authorizations and decision-making documents. The law also strengthens stakeholder engagement.

Texas Projects

As for projects in Texas, the law:

  • Authorizes a navigation feasibility study of the Trinity River and Tributaries near Liberty, Texas, as defined in the USACE 2017 and 2018 reports to Congress;
  • Authorizes a flood risk management feasibility study associated with West Cell Levee, Irving, Texas, as defined in the USACE 2017 and 2018 reports to Congress;
  • Directs the USACE to “expedite the completion of studies for flood damage reduction, hurricane and storm damage reduction, and ecosystem restoration in the coastal areas of Texas that are identified in the interim report due to be published in 2018 that describes the tentatively selected plan developed in accordance with section 4091 of the Water Resources Development Act of 2007;”
  • Directs USACE to prepare a report on the status of the implementation of a water supply contract at Wright Patman Lake, Texas; and,
  • Directs USACE to “expeditiously carry out any project for flood risk management or hurricane and storm damage risk reduction authorized as of the date of enactment of this Act to be carried out by the Secretary in Texas, Florida, Georgia, Louisiana, South Carolina, the Commonwealth of Puerto Rico, or the United States Virgin Islands.”

Please note that “authorizations” in federal law, just provide direction and permission to take action, but without any funding.  In federal water resources programs, Congress must then decide to “appropriate” funding to actually complete authorized projects.

Did you notice anything interesting in this year’s WRDA? Leave a comment if you did.

Oh, and Happy Halloween!

Financial Impact of Atlas 14, Volume 11

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Someone recently asked me if I could estimate the financial impacts to the Houston area from the release of Atlas 14, Volume 11, Version 2.0. The short answer is no, but I can write a blog post with a rough qualitative and directional assessment of impacts!

Flood damage reduction infrastructure will need to be bigger to achieve the 1% annual chance risk level we have generally settled upon. This will mean that pipes or structures that carry the runoff that is generated from the 1% annual chance, 24-hour rainfall will be larger. Thankfully pipes, culverts, and earthen ditches don’t increase in cost linearly with their carrying capacity, but they do tend to get more pricey as they get bigger.

As you drive around the Houston/Harris County region you might sometimes notice concrete “notches” in the banks of some of the bayous or “flumes” that connect the bayou to a detention basin.  You might also see the same type of structure in smaller detention basins in and around neighborhoods.  Those structures allow the runoff from larger storm events to flow to or from the bayou and the channel with less chance of erosional damage.  Because they often are designed to carry the runoff from larger storms, they will be larger if they are designed to carry the flow arising from the updated rainfall stats. Here’s a screenshot of one of these structures from Google Maps:

Pipes that carry smaller storms may not increase in size that much or at all, because the Atlas 14 information did not change the smaller, more frequent rain event depths that much. For example, the 50% annual chance, 24-hour event reported for the center of Harris County near Houston in “Technical Paper No. 40: Rainfall Frequency Atlas of the United States, For Durations From 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years” (affectionately known as “TP-40”) (1961) is somewhere in the 5.1 to 5.2 range on the national scale map (screenshot below) and the same event in Atlas 14 is reported as 5.11 inches; so they are pretty close.  This means that the smaller neighborhood storm sewer pipes that are designed to carry runoff from these frequent, smaller events, might be able to stay the same size and, therefore, cost about the same.

One of my co-workers evaluated a hypothetical 160-acre green-field development site to see if detention ponds would need to be larger as a result of the initial draft release of Atlas 14, using a storm event depth of 17.7 inches in 24 hours. The evaluation showed that both the pre-development and the post-development flows increase, so the difference was not large enough to exceed the applicable regulatory minimum detention rates of 0.50 acre-feet or 0.65 acre-feet (Houston and Harris County). The results of this evaluation are shown below.

This suggests that most detention basins will still be sized to provide between 0.5 and 0.65 acre-feet per acre of development because most local governments won’t let folks install smaller ones.  This will have to be evaluated for more sites to be sure, but that’s what we saw for the one site we evaluated.

Regulatory floodplains will be remapped over the next three to four years using the more accurate rainfall depths. Harris County Flood Control District is already moving forward with this work.  This will enlarge them and more existing structures will then be located in the regulatory floodplain. This will increase the cost of insurance premiums to obtain coverage for many buildings and facilities. Approximately 37% of the county is inside one of the regulatory floodplains defined by the National Flood Insurance Program (NFIP). These include areas that have greater than a 1% annual chance riverine flooding, greater than 0.2% annual chance of riverine flooding, or greater than a 1% annual chance of coastal flooding.  The updated rainfall data will increase the area of the county that is in a regulatory floodplain by some amount.

Based on the elements above, it appears that the financial impact of the publication of the rainfall statistics update will be less significant than the financial impact of the floodplain map updates coming in a few years.

If you have other thoughts about cost or financial impacts, please leave a comment.

Version 2.0 of Atlas 14, Volume 11 (Sept 2018)

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On September 27, 2018 Version 2.0 of the National Oceanic and Atmospheric Administration’s Atlas 14, Volume 11 was released and a press conference was held. As I’ve described in previous posts, Volume 11 provides us with a more accurate picture of the annual probability of rainfall of various depths and durations across the state of Texas.  These data were last updated across the entire state in 1961.  Volume 11 was created by evaluating rainfall records as early as the 1870’s up through 2017.

As I written previously, the depth of 1% annual chance, 24-hour rainfall is a key design storm we use for drainage and flood risk reduction infrastructure in the Harris County area.  Prior to Volume 11, this key design storm was about 12.4-inches in western Harris County and about 13.5-inches in southeastern Harris County, as shown below.

After Volume 11’s release the new 1% annual chance, 24-hour duration rainfall map for Harris County looks like this:

We used to believe that the 1% annual chance, 24-hour storm was 2.6-inches to 4.5-inches smaller than it actually is. This was because the rainfall records did not span as long a period as they do now. Our uncertainty bands have gotten a bit narrower and the overall magnitude of most storm frequencies and durations have increased.  The actual depth of the 100-year, 24-hour duration rainfall ranges from 21% to 33% higher than we thought.

The updated information is presented by NOAA using a web-based map interface, located here: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html. It allows anyone to double click on the map at any point of interest and instantly get both a tabular and graphical display of the rainfall stats.

Here’s how it works for my neighborhood in west Houston.

This first image is from the map interface. I have zoomed into my part of town and have double-clicked on the map near my house. The red cross-hairs show the click point.

After double-clicking the following information appears below the map:

This shows that my house has a 1% annual chance of receiving 16.5 inches (or more) of rain in 24-hours.  If you read the fine print you will see that there is a 90% chance that the actual rainfall depth for this 1% annual chance, 24-hour storm ranges will be somewhere between 11.6 inches and 23.5 inches, because of the uncertainty of our statistical estimate.  The website also provides a graphic version of the same data:

The website also provides a graph that illustrates the confidence bands around the estimated rainfall depth. The figure below shows confidence range for all 24-hour storms.  The green line shows the upper limit of the 24-hour rainfall depth and the red line shows the lower limit. There is 90% chance that the “true” rainfall depth will fall in between those values (for any given recurrence interval).  Please recall that “recurrence interval” is the inverse of annual probability.  So the 1,000-year storm has a 1/1,000% chance of occurring in any given year.  If you convert the fraction to a decimal, that’s a 0.001% annual chance. Notice how that is not zero?

See how the confidence range grows for the larger, more rare events?  The 100-year recurrence interval corresponds to the 1% annual chance event. The math tells us that there is 90% chance that the 1% annual chance, 24-hour event is somewhere between 11.6 inches and 23.5 inches. The corollary is there is 10% chance that the real depth is outside that range.

The expanding confidence interval – the gap between the green line and red line – makes sense because we have statistically evaluated rainfall records that only go back to, at best the 1870s, and, at worst 1970’s.  This is a period of record of only between 47 and 148 years.  So our certainty about 500-year or 1,000-year recurrence interval events is low.

In my next post I will be providing some thoughts about the cost impacts of this refined view of our rainfall statistics.

A Response to “Remember the Villains of Hurricane Harvey”

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On September 2, 2018, the Houston Chronicle published “Remember the Villains of Hurricane Harvey” as their lead editorial. The byline was listed simply as “The Editorial Board.”  The editorial board is currently composed of the following people (just in case you wondered):

  • John McKeon, Publisher.
  • Jack Sweeney, Chairman.
  • Lisa Falkenberg, Opinion Editor.
  • Evan Mintz, Deputy Opinion Editor.
  • Jim Newkirk, Outlook Editor.
  • Andrea Georgsson, Sunday Outlook Editor.
  • Harold Jackson, Editorial Writer.
  • Andrea White, Editorial Writer.

STAR WARS meme prepared by m. bloom paraphrasing the lead editorial only slightly.

The editorial called for Houstonians to remember villainous developers “who saw nothing wrong with building neighborhoods inside the flood pools behind Addicks and Barker reservoirs, and then kept the risks to themselves.”

First, some definitions of the word “villain,” to place the editorial in the proper context:

Dictionary.com: Villain: (1) a cruelly malicious person who is involved in or devoted to wickedness or crime; scoundrel; (2) a character in a play, novel, or the like, who constitutes an important evil agency in the plot.

Merriam-Webster: Villain: (1) a character in a story or play who opposes the hero; (2) a deliberate scoundrel or criminal; (3) one blamed for a particular evil or difficulty. 

Cambridge: Villain (1) a bad person who harms other people or breaks the law; (2) a cruel or evil character in a book, play, or film.

Let’s break this down one bit at a time.

The developers (and the professional engineers who designed their developments) invested private capital to provide affordable housing on privately-owned land, with an annual risk of flooding of much less than 1%, which is the consensus standard used by all communities in the United States to design and locate new homes and other structures.  Engineers designed streets, lights, traffic controls, water systems, wastewater systems, and drainage facilities that functioned perfectly from the day they were built until Harvey arrived.

I believe that the engineers designing these neighborhoods did so in compliance with Sections 137.51 through 137.55 of the rules promulgated by the Texas Board of Professional Engineers under the legal authority of the Texas Engineering Practice Act.  These rules require engineers “to safeguard, life, health and property, to promote the public welfare” as well as “to protect the health, safety, property, and welfare of the public.” Please note that this rule does not require engineers to design neighborhoods with zero risk of flooding.

Why do I believe that the developers and engineers acted ethically and appropriately? Simply because the health, safety, property, and welfare of the residents were safeguarded.

Because the potable water provided to the residents was clean and healthy every day until Harvey arrived. Ever try to raise a family without potable water?

Because sanitary wastewater was collected from every home, treated, and properly discharged into nearby bayous every day until Harvey arrived.  Ever try to live in a home without a toilet or proper wastewater facilities?

Because none of the homes fell down on their owners every day until Harvey arrived and even during Harvey.

Because the roads carried traffic every day until Harvey arrived. What if there were no roads? Because the traffic lights and street lights worked every day until Harvey arrived. What if there were no lights?

Because rainwater from storm after storm was carried downstream from the developments without flooding anyone every day until Harvey arrived.  Because the systems worked.

All of these critical public health, safety, and welfare facilities were designed by ethical engineers with private funding from developers. All of the facilities and homes were made possible by the developers who invested private capital to create desirable places for people to live with a low and acceptable level of risk.

We’ve already decided to push our inundation risk exposure down for new construction in the City of Houston and Harris County to significantly less than 0.2% per year. Note that a finished floor elevation placed 2 feet higher than the 500-year flood elevation has an annual chance of inundation of much less than 0.2% per year in most places in this region.

But, if we collectively wish to retrofit existing homes and businesses in our region that are exposed to higher annual risk levels than 0.2% or 1% (think Hunting, White Oak, Brays – but certainly not neighborhoods in the Addicks or Barker flood pools) we need to have an open conversation about the desired risk level, the costs to achieve it, and the benefits realized.

We should avoid villainizing folks that built communities and facilities that worked and that beat the desired risk levels established at the time those facilities were built.

The villains pictured below are not developers.

A still from the silent comedy, Barney Oldfield’s Race for a Life MACK SENNETT/PUBLIC DOMAIN